1. Field of the Invention
The present invention relates to nonlinear optical devices for converting laser radiation wavelength by means of quasi-phase-matched, parametric interaction or sum/harmonic frequency generation. More particularly, the invention uses a zig-zag beam path in an optically flat and parallel slab, in which each total internal reflection provides the necessary phase shift to accomplish phase matching for each leg of the zig-zag path.
2. Description of the Background Art
Quasi-phase-matching (QPM) is a known technique for realizing efficient generation of radiation from coherently interacting waves in a nonlinear optical medium that lack birefringence. Prior structures for achieving quasi-phase matching involves optical devices including a series of two alternating optical layers. In one class of approaches, the phase mismatch in each layer is set to an odd multiple of .pi. radians, while the sign of the nonlinear coefficient reverses in alternating layers. The sign reversal is equivalent to an additional phase shift of .pi. radians. These two conditions are required to maximize power conversion in each layer and to achieve cumulative growth in successive layers. This approach is used in periodically poled lithium niobate crystal fibers, stacks of discrete plates at Brewster angle, and diffusion-bonded plates. Other approaches use a non-interacting material for the second layer that simply provides phase shifts needed to reach a combined phase mismatch of an integral multiple of 2.pi.. Examples of this approach include periodically structured waveguides.
Representative art in wavelength conversion technology include the following references:
Minemoto et al. U.S. Pat. No. 5,167,000 issued Nov. 24, 1992 entitled OPTICAL WAVELENGTH CONVERTER discloses a nonlinear optical device which uses non-linear optical materials that transmit a fundamental wave and generated higher harmonics, and have optical absorption maximums based on an electronic transition between wavelengths of the fundamental wave and the generated higher harmonics. By emitting higher harmonics having a wavelength within an optical transmission region between wavelengths of the two optical absorption maximums, a device with high conversion efficiency can be realized.
Katoh U.S. Pat. No. 5,049,762, issued Sep. 17, 1991 entitled OPTICAL WAVELENGTH CONVERTER SYSTEM discloses an optical wavelength converter system that includes a light source for emitting a fundamental wave, a fiber or optical waveguide type optical wavelength converter device for converting the wavelength of the fundamental wave and emitting a wavelength converted wave, and an optical system for introducing said fundamental wave into the optical wavelength converter device. The optical wavelength converter device includes an element made of an organic nonlinear optical material and covered with a cladding layer which has a smaller refractive index that the refractive index of said element.
Nishio et al U.S. Pat. No. 4,997,244 issued Mar. 5, 1991 entitled OPTICAL WAVELENGTH CONVERTING DEVICE AND MANUFACTURING METHOD THEREOF discloses an optical wavelength converting device that includes a substrate, and a waveguide layer of nonlinear organic material formed on one major surface of the substrate and having a thickness tapered along one axis parallel to the major surface, in which a waveguide with a desired thickness can be selected in a direction normal to the axis.
Okazaki et al. U.S. Pat. No. 4,893,888 issued Jan. 16, 1990 entitled OPTICAL WAVELENGTH CONVERTER DEVICE discloses an optical wavelength converter device that has a waveguide of a nonlinear optical material disposed in cladding and having a refractive index lower than the refractive index of the cladding for converting a fundamental guided through the waveguide into a second or third harmonic and radiating the second or third harmonic into the cladding. The nonlinear optical material comprises an organic nonlinear optical material having a maximum light absorption coefficient at a wavelength close to the wavelength of at least one of the fundamental and the second harmonic.
Jacques et al U.S. Pat. No. 3,832,567 issued Aug. 27, 1974 entitled, TRAVELLING WAVE FREQUENCY CONVERTER ARRANGEMENT relates to travelling wave frequency converter arrangements based on the harmonic generation. The converter in accordance with the invention comprises a harmonic generation interface obtained by bringing together a metal film and an optical waveguide layer whose thickness is such that the phase velocities of the fundamental and harmonic frequency radiations transmitted are substantially matched with one another. Optical coupling means are associated with the optical waveguide and electrical means may be provided for altering the phase velocity matching.
Representation art in harmonic generating technology include the following references:
Penner et al. U.S. Pat. No. 5,150,446 issued Sep. 22, 1992 entitled CONVERSION EFFICIENCY SECOND HARMONIC GENERATOR discloses an optical article comprised of a support including a portion adjacent one major surface which is transparent to the electromagnetic radiation sought to be propagated, an organic layer unit capable of converting a portion of polarized electromagnetic radiation of a selected wavelength to its second harmonic wavelength, means for optically coupling into said organic layer unit polarized electromagnetic radiation of a selected wavelength in its zero order transverse magnetic mode, and means for receiving from the layer unit a portion of the electromagnetic radiation in the form of a first order transverse magnetic mode.
Takano et al. U.S. Pat. No. 5,073,725 issued Dec. 17, 1991 entitled OPTICAL HARMONIC GENERATOR discloses an apparatus wherein a metal surface plasmon is excited in a multilayered film composed of thin metal films and a thin dielectric film, by light having a propagation constant larger than that of the incident light emitted from an optical component such as a prism. Due to an extremely strong alternating electric field produced by the electric field enhancement effect of the metal surface plasmon, a harmonic is generated from a nonlinear optical crystal.
Schildkraut et al. U.S. Pat. No. 5,058,970 issued Oct. 22, 1991 entitled QUASI-PHASE MATCHING OPTICAL WAVEGUIDE discloses a quasi-phase matching optical waveguide for producing a second harmonic of an internally propagated polarized laser beam. The waveguide is comprised of at least one array of laterally spaced transparent electrodes in direct contact with a transmission medium containing similarly polar aligned organic molecular dipoles in overlying areas. The transparent electrodes and overlying areas of the transmission medium are each of the same width and spacing.
Khanarian et al. U.S. Pat. No. 4,971,416 issued Nov. 20, 1990 entitled POLYMERIC WAVEGUIDE DEVICE FOR PHASE MATCHED SECOND HARMONIC GENERATION discloses a parametric frequency converting device which comprises a thin film of a polymeric medium which exhibits second order nonlinear optical response, and the device has heat control means for temperature tuning of the waveguide medium to phase match the propagation constants of fundamental and second harmonic light beams. In a preferred device the waveguiding medium has a spatial quasi structure for quasi-phase matching of the propagating wave energy, and optionally has a set of electrodes for application of a direct current electric field to the waveguiding medium.
Araki et al U.S. Pat. No. 4,907,850 issued Mar. 13, 1990 entitled APPARATUS FOR PERIODICALLY GENERATING SECOND HARMONIC discloses an apparatus for periodically generating a second harmonic light comprises: a light source; and means for converting a fundamental wavelength light emitted from said light source to a second harmonic light, including optical wave guide means having a light propagation area and means for periodically varying a refractive index of the light propagation area.
References that disclose waveguides that provide frequency doubling include:
Rikken et al. U.S. Pat. No. 5,151,965; Lawandy U.S. Pat. No. 5,028,109; Meijer et al. U.S. Pat. No. 5,006,729; and Khanarian et al. U.S. Pat. No. 4,865,406.